1. Field of the Invention
This invention describes materials formed from pyrrole or copolymers of pyrrole exhibiting high surface area.
2. Description of the Art
It is known that polymers of pyrrole and copolymers of pyrrole may be obtained as electronically conducting organic polymers. Such materials have been known to have conductivities exceeding 1 ohm.sup.-1 cm.sup.-1. It is further known that materials of a similar class include polyacetylene, poly-p-phenylene, poly-p-phenylene sulfide, polyaniline and poly (2,5 thienylene).
Pyrrole black, a polymeric powdered material formed by oxidizing pyrrole in homogeneous solution (e.g. with H.sub.2 O.sub.2 has been known for many years. Gardini, Adv. Heterocycl. Chem, 15, 67(1973). An electrochemical method of producing polypyrrole as a powdery film on an electrode has been reported. A. Dall' olio et al., Comp. Rend., 433, 267c (1968). Electropolymerization to produce polymer films from pyrrole has been reported. Diaz et al, J. Chem. Soc, Chem. Comm., 635, (1979)(hereinafter Diaz (I)); Diaz et al, J. Chem. Soc.I, Chem. Comm., 397, (1980) (hereinafter Diaz (II)); Diaz, Chemica Scripta, 17, 45, (1981) (hereinafter Diaz (III)); and Kanazawa et al., J. Chem. Soc., Chem. Comm., 954 (1979) (hereinafter Kanazawa (I)). Electropolymerized polypyrrole from substituted pyrroles has been reported. Diaz (II); Diaz et al., J. Electroanal Chem., 129, 115, (1981) (hereinafter Diaz (IV)); Diaz et al., J. Electroanal Chem., 130, 181, (1981) (hereinafter Diaz (V)). Co-polymer films produced by electropolymerizing mixtures of pyrrole and substituted pyrrole have also been reported. A mixture of pyrrole and N-methyl pyrrole has been polymerized and it is believed that both monomers are incorporated into the polymer. Diaz (II); Diaz, Proc. Int. Conf. on Low Dimensional Synthetic Metals Chemica Scripta, 17, 0000, (1981) (hereinafter Diaz (VI; and Kanazawa et al J. Synth Metals, 4, 119, (1981) (hereinafter Kanazawa (II)).
Polypyrrole is electronically conducting in the charged or oxidized state (black), and is produced in this state by electropolymerization. When completely reduced to the neutral or discharged state (yellow), it is an electronic insulator. The electropolymerized polypyrrole is produced in the oxidized, i.e. conductive, state and (unlike other conducting polymers such as polyacetylene) does not require any subsequent chemical or electrochemical treatment to increase its conductivity above 1 ohm.sup.-1 cm.sup.-1. A counter-anion is incorporated into the material during the electropolymerization process to balance the positive charge on the polymer backbone. Diaz (III).
Polypyrrole can also be electropolymerized as a continuous film on electrodes. The highest electronic conductivities reported for the continuous films are of the order of about 100 ohm.sup.-1 cm.sup.-1. Diaz (I); Kanazawa et al., Syn. Metals, 329, (1980) (hereinafter Kanazawa (III)). These conductivities can be orders of magnitude lower depending on the counter-anion incorporated.
A large number of counter-anions have been used, including BF.sub.4.sup.-, PF.sub.6.sup.-, AsF.sub.6.sup.-, ClO.sub.4.sup.-, HSO.sub.4.sup.-, CF.sub.3 SO.sub.3.sup.-, CH.sub.3 C.sub.6 H.sub.4 SO.sub.3.sup.-, CF.sub.3 COO.sup.-, HC.sub.2 O.sub.4.sup.-, Fe(CN).sub.6.sup.3-. Diaz (IV); Diaz (VI); Kanazawa (I); and Noufi et al, J. Electrochem. Soc., 128 2596, (1981).
N-substituted pyrroles have been polymerized, including methyl, ethyl, n-propyl, n-butyl, isobutyl and phenyl and substituted phenyl pyrroles. Diaz (IV); Diaz et al., Electrochemical Society Extended Abstracts, Vol. 82-1. Abstract No. 617, (1982) (hereinafter Diaz (VII)). These materials have reported electronic conductivities orders of magnitude lower than polypyrrole itself. Kanazawa (II); Diaz (VII). It has been reported that beta-substituted pyrroles such as 3,4 dimethyl pyrrole have been polymerized. Gardini, supra.
The polymers have been prepared to date with non-aqueous solvents, typically acetonitrile, (Diaz (I); Diaz (II); and Diaz (VI)), containing a dissolved salt which provides the counter-anion. It is known that the physical properties of the resulting films are sensitive to the formation conditions. For example, in acetonitrile, small traces of water in the solvent produce a film with a smoother surface than that produced in anhydrous acetonitrile. Diaz (VI). The polypyrrole tetrafluoroborate films produced by Diaz et al. are continuous, space-filling and very poorly crystalline with a density of 1.48 g cm.sup.-3. Kanazawa (III).
Polypyrrole films are thermally stable at room temperature and are insoluble in common solvents. Diaz (I); Kanazawa (II); and Tourillon et al., Electrochemical Society Extended Abstracts, Vol. 82-1, Abstract No. 618, (1982). Polypyrrole in the oxidized form is reported to be chemically stable in ambient conditions of O.sub.2 and moisture for several months. Diaz et al., J. Electroanal. Chem., 121, 355, (1981) (hereinafter Diaz (VIII)); Watanabe et al, Bull. Chem. Soc. Jpn., 54, 2278, (1981). Polypyrrole fluoroborate films have been shown to be unstable under oxidative conditions such as potentials greater than +0.6 v. (SCE) or in the presence of halogens. Bull et al, J. Electrochem. soc., 129, 1009. (1982).
Polypyrrole can be driven repeatedly between the conducting and non-conducting state. Bull et al, supra; Diaz et al, "Conducting Polymers", Polymer Sci. & Technology, p. 149 et. seq., Plenum Press, N.Y., (1981) (hereinafter Diaz (IX)). Rapid complete switching is reported to require the use of thin films (i.e., less than about 0.1 micrometer) and switching is difficult for thicknesses greater than about 1 micrometer. Diaz (II); Diaz (IX). It has been shown that although a film may contain BF.sub.4.sup.- counter-anions when it is formed
(i.e., is the charged state), BF.sub.4.sup.- is no longer present in the film when it is in the neutral (i.e., reduced or discharged) state. Diaz (IV). It has been suggested that both the anion and the cation of the electrolyte salt affect ion diffusion during reduction and oxidation of polypyrrole films. Diaz (IV).
Polymers have been produced with different degrees of oxidation, depending on the anion and/or substituents. Most of these materials have degrees of oxidation around 0.25 (i.e., one quarter of the polymer rings are oxidized). In an article entitled "Proton Modification of Conducting Polypyrrole", J. Phys. Chem. Solids, Volume 48, No. 4, pages 427-432, Inganaes et al reported that the deprotonation of polypyrrole may be accomplished through the use of caustic.
It is not currently known that a polymer of pyrrole or copolymer of pyrrole can be made having a significantly higher surface area. It is highly desirable to have a high surface polymer which is essentially nonreactive as it may be used as a catalyst support such as for a Friedel Crafts catalyst or an oxygen reduction electrode.
It has been found that by treating the polymers, for instance, which are obtained according to U.S. Patent Ser. No. 460,695 filed on Jan. 24, 1983 that suitable precursors for high surface area polymers may be obtained. Specifically, it has been found that the processing in the aforementioned application allows for the polymers of pyrrole or copolymers of pyrrole to be deprotonated thereby rendering an extremely high surface area polymer which is suitable as a catalyst support.
Throughout the specification and claims, percentages and ratios are by weight; temperatures are in degrees Celsius and pressures are in KPa over ambient unless otherwise indicated. The references cited herein are specifically incorporated by reference to the extent applicable.